The Relationships of Structure to Properties in Graphite Fibers. Part 2
Final rept. May 1971-Apr 1972
RENSSELAER POLYTECHNIC INST TROY NY MATERIALS DIV
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Three dimensional structural models of carbon fibers prepared from different precursors and representing a wide modulus range have been developed. Commercially available carbon fibers and the organic fibers such as PRD 49 have also been studied. Results show that increasing fiber modulus is accompanied by development of thicker, straighter, more crystalline, and better aligned ribbons of graphite. Quantitative optical measurements on fibers indicate a very steep modulus gradient is present in fibers from skin to core such that when fibers are loaded in tension, most of the load is borne by the skin. Polyacrylonitrile PAN fibers experience an inflection in density in the 30-50 Msi. range while the density of hot stretched fibers is uniformly proportional to modulus. Basal spacing and stack height fluctuate in the region of the density anomaly for the PAN fibers inferring a degree of structural disordering which may be related to the strength anomaly. In addition, the preferred orientation versus modulus relations for PAN AND RAYON BASE FIBERS IS DIFFERENT IMPLYING CONSTANT STRAIN CONDITIONS IN PAN. Modulus is directly proportional to preferred orientation in organic fibers, and compressive failure is caused by buckling of polymer chains.